(1) Field of the Invention
The present invention relates to a voltage non-linear resistor having a large non-linear coefficient, which is composed of a sintered body comprising zinc oxide as the main ingredient.
(2) Description of the Prior Art
A voltage non-linear resistor composed of a sintered body obtained by adding an oxide such as bismuth oxide, manganese oxide, cobalt oxide, antimony oxide, nickel oxide, chromium oxide, silicon oxide, boron oxide, lead oxide, magnesium oxide or aluminum oxide to zinc oxide as the main ingredient and shaping and sintering the mixture, or of a sintered body obtained by adding an oxide such as lanthanum oxide, praseodymium oxide, samarium oxide, neodymium oxide, cobalt oxide or manganese oxide to zinc oxide as the main ingredient and shaping and sintering the mixture has been used widely as a voltage stabilizing element, a surge absorber or an arrester. When a voltage non-linear resistor of this type is used as a high voltage surge absorber or arrester, the side face is protected with glass so as to prevent the face flashover. Such device is disclosed in the specification of U.S. Pat. No. 3,905,006 to Matsuoka et al.
The known voltage non-linear resistor comprises a sintered body comprising zinc oxide as the main ingredient, electrodes formed on confronting main faces, respectively, and a glass layer formed on the side face connecting the main faces to each other. Since a high-resistance glass layer is formed on the side face, in this element, the face flashover hardly takes place. This element is also advantageous in that since the side face of the element is smooth, it is hardly contaminated.
A voltage non-linear resistor including a high-resistance intermediate layer comprising zinc antimonate and zinc silicate as the main ingredient, which is formed below the above-mentioned glass layer, has an especially high resistance to the face flashover.
In the conventional voltage non-linear resistors, a lead borosilicate glass having a thermal expansion coefficient of 50.times.10.sup.-7 to 100.times.10.sup.-7 /.degree.C. or a zinc borosilicate glass having a similar thermal expansion coefficient, or a glass formed by incorporating titanium oxide, aluminum oxide or copper oxide in such glass has been used as a coating glass layer because of the following characteristic properties:
(1) High heat cycle resistance. PA1 (2) Good moisture resistance. PA1 (3) Easiness in handling. PA1 (1) The insulating strength of glass is not degraded. PA1 (2) The dispersibility in glass is very good and combustion of the binder is uniformly accomplished. PA1 (3) Barium oxide exerts a sufficient catalytic action at a temperature lower than 400.degree. C.
For coating the side face of a resistor with such glass, such glass powder is mixed with an organic binder to form a glass plate, the paste is applied to the side face of the resistor and is then heated and baked in an oxidative atmosphere at 400.degree. to 650.degree. C.
However, a resistor having the side face coated according to the above-mentioned method is defective in that the leak current in the low voltage region is larger than in a resistor free of a glass coating and the linearity is inferior. For example, the non-linear coefficient .alpha. is 50 in a resistor before the glass coating, but it is reduced to 20 or less after the glass coating.